How to Use This Page
Welcome to the Basic Concepts of Organic Reactions exercise solutions for Class 11 Chemistry (Tamil Nadu State Board). Here is how to get the most from this page:
1. Attempt every question on your own first — do not jump straight to the answers. 2. Compare your answer with the solution provided. If they differ, trace through the explanation step by step. 3. Re-work mistakes — write the corrected answer in your own words; this builds deeper understanding. 4. Use the Key Concepts section for a quick recap before exams.
Key Concepts (Quick Recap)
Types of Organic Reactions
| Reaction Type | Description | Example |
|---|---|---|
| Substitution | One atom/group is replaced by another | CH₄ + Cl₂ → CH₃Cl + HCl |
| Addition | Atoms/groups are added across a double/triple bond | CH₂=CH₂ + H₂ → CH₃–CH₃ |
| Elimination | Atoms/groups are removed to form a double/triple bond | C₂H₅OH → CH₂=CH₂ + H₂O |
| Rearrangement | Molecule rearranges to form an isomer | CH₃CH₂CH₂CH₃ → (CH₃)₂CHCH₃ (rare, specific conditions) |
Electrophile vs Nucleophile
| Feature | Electrophile | Nucleophile |
|---|---|---|
| Definition | Electron-seeking (electron-poor) species | Electron-donating (electron-rich) species |
| Charge | Positively charged or neutral with empty orbital | Negatively charged or neutral with lone pair |
| Role | Accepts an electron pair | Donates an electron pair |
| Examples | H⁺, NO₂⁺, SO₃, BF₃, AlCl₃ | OH⁻, CN⁻, NH₃, H₂O, CH₃O⁻ |
| Behaviour | Attacks electron-rich centres | Attacks electron-deficient centres |
Inductive Effect vs Resonance (Mesomeric) Effect
| Feature | Inductive Effect (I) | Resonance/Mesomeric Effect (M) |
|---|---|---|
| Bonds involved | Sigma (σ) bonds only | Pi (π) bonds and conjugated systems |
| Nature | Permanent polarization | Delocalisation of π-electrons |
| Range | Short-range (dies out after 3-4 carbons) | Long-range (through entire conjugated system) |
| +I effect | Electron-donating groups (alkyl, –CH₃) | +M: Electron-donating by resonance (–OH, –NH₂, –OR) |
| –I effect | Electron-withdrawing groups (–NO₂, –CN, –Cl) | –M: Electron-withdrawing by resonance (–NO₂, –CN, –CHO) |
| Effect on acidity | –I groups increase acidity; +I groups decrease acidity | –M groups increase acidity; +M groups decrease acidity |
Free Radical Mechanism — Steps
| Step | Description | Example (Chlorination of Methane) |
|---|---|---|
| Initiation | Bond breaks homolytically to form free radicals (requires UV light or heat) | Cl₂ → 2 Cl· (homolytic fission) |
| Propagation | Free radical attacks a molecule to form a new radical (chain reaction) | Cl· + CH₄ → HCl + ·CH₃; ·CH₃ + Cl₂ → CH₃Cl + Cl· |
| Termination | Two radicals combine to form a stable molecule (chain ends) | Cl· + Cl· → Cl₂; ·CH₃ + Cl· → CH₃Cl; ·CH₃ + ·CH₃ → C₂H₆ |
Electron-Displacing Effects Summary
| Effect | Symbol | Direction | Groups | Impact on Reactivity |
|---|---|---|---|---|
| +Inductive | +I | Pushes e⁻ away from itself | –CH₃, –C₂H₅, –CH(CH₃)₂ | Increases nucleophilicity at ortho/para positions |
| –Inductive | –I | Pulls e⁻ towards itself | –Cl, –NO₂, –CN, –COOH | Decreases nucleophilicity; increases acidity |
| +Mesomeric | +M | Donates e⁻ via resonance | –OH, –NH₂, –OR, –NHR | Activates ring for electrophilic substitution |
| –Mesomeric | –M | Withdraws e⁻ via resonance | –NO₂, –CN, –CHO, –COOH | Deactivates ring for electrophilic substitution |
Homolytic vs Heterolytic Fission
| Feature | Homolytic Fission | Heterolytic Fission |
|---|---|---|
| Definition | Bond breaks equally; each atom gets one electron | Bond breaks unequally; both electrons go to one atom |
| Products | Two free radicals | A cation and an anion |
| Example | Cl₂ → 2 Cl· | CH₃–Cl → CH₃⁺ + Cl⁻ |
| Conditions | UV light, heat, peroxides | Polar solvents, ionic conditions |
I. Choose the Best Answer (MCQs)
II. Fill in the Blanks
1. A species that seeks electrons is called an electrophile.
2. The permanent polarization of a sigma bond due to electronegativity difference is called the inductive effect.
3. In heterolytic fission, the bond breaks unequally, producing a cation and an anion.
4. The electron-donating groups that push electron density away from themselves exhibit the +I effect.
5. The delocalisation of π-electrons through conjugated systems is called the resonance or mesomeric effect.
6. In a free radical mechanism, the step where two radicals combine to form a stable molecule is called termination.
7. NH₃ acts as a nucleophile because it has a lone pair on nitrogen.
8. The –NO₂ group exhibits the –I and –M effect, withdrawing electrons both inductively and by resonance.
9. In the chlorination of methane, the propagation step produces HCl and methyl radical (·CH₃).
10. Hyperconjugation involves the delocalisation of sigma (σ) electrons from adjacent C–H bonds.
11. The stability order of carbocations is: 3° > 2° > 1° (tertiary > secondary > primary).
12. The order of reactivity for nucleophilic substitution is: 1° > 2° > 3° for SN2 mechanism.
13. The electromeric effect is a temporary polarization of π-electrons in the presence of a reagent.
14. BF₃ is an electrophile because boron has an incomplete octet.
15. The species formed by homolytic fission of a bond are called free radicals.
III. True or False (with Corrections)
IV. Match the Following
Set 1: Reagents and Types
| Column A | Column B |
|---|---|
| 1. OH⁻ | (a) Electrophile |
| 2. NO₂⁺ | (b) Nucleophile |
| 3. BF₃ | (c) Electrophile |
| 4. NH₃ | (d) Nucleophile |
| 5. AlCl₃ | (e) Electrophile |
1. OH⁻ → (b) Nucleophile 2. NO₂⁺ → (a) Electrophile 3. BF₃ → (c) Electrophile 4. NH₃ → (d) Nucleophile 5. AlCl₃ → (e) Electrophile
Set 2: Effects and Groups
| Column A | Column B |
|---|---|
| 1. +I effect | (a) –NO₂, –CN |
| 2. –I effect | (b) –CH₃, –C₂H₅ |
| 3. +M effect | (c) –NO₂, –CHO |
| 4. –M effect | (d) –OH, –NH₂ |
1. +I effect → (b) –CH₃, –C₂H₅ 2. –I effect → (a) –NO₂, –CN 3. +M effect → (d) –OH, –NH₂ 4. –M effect → (c) –NO₂, –CHO
V. Answer Briefly (2-mark)
VI. Answer in Detail (5-mark)
Inductive Effect
The inductive effect is the permanent polarization of a sigma bond caused by the electronegativity difference between two atoms. The electron density shifts along the chain of sigma bonds, but the effect diminishes rapidly with distance.
Types:
- +I Effect (Electron-donating): Groups that push electron density away from themselves. Alkyl groups (–CH₃, –C₂H₅, –CH(CH₃)₂) show the +I effect.
- –I Effect (Electron-withdrawing): Groups that pull electron density towards themselves. Examples: –Cl, –NO₂, –CN, –COOH, –F.
Application — Effect on Acidity:
Resonance (Mesomeric) Effect
The resonance effect is the delocalisation of π-electrons through conjugated systems (alternating single and double bonds, or a lone pair adjacent to a π-bond). It stabilises the molecule by distributing electron density over multiple atoms.
Types:
- +M Effect (Electron-donating by resonance): Groups that donate electrons through resonance into the conjugated system. Examples: –OH, –NH₂, –OR, –NHR.
- –M Effect (Electron-withdrawing by resonance): Groups that withdraw electrons through resonance from the conjugated system. Examples: –NO₂, –CN, –CHO, –COOH.
Comparison Table
| Feature | Inductive Effect | Resonance Effect |
|---|---|---|
| Bonds involved | Sigma (σ) | Pi (π) and conjugated systems |
| Nature | Permanent polarization | Delocalisation of π-electrons |
| Range | Short (3-4 carbons) | Long (entire conjugated system) |
| Depends on | Electronegativity difference | Presence of conjugation |
| Effect type | +I or –I | +M or –M |
Step 1: Initiation
The reaction begins when UV light (or heat) provides enough energy to break the Cl–Cl bond homolytically. Each chlorine atom retains one electron, forming two chlorine free radicals.
This is the energy-absorbing step that starts the chain reaction. The bond dissociation energy of Cl₂ is about 242 kJ/mol.
Step 2: Propagation
The chlorine radical attacks a methane molecule, abstracting a hydrogen atom to form HCl and a methyl radical.
The highly reactive methyl radical then attacks a chlorine molecule, producing chloromethane and regenerating a chlorine radical.
The regenerated Cl· radical can attack another CH₄ molecule, continuing the chain. This is why it is called a chain reaction — one initiation event leads to many product molecules.
Step 3: Further Chlorination (if excess Cl₂ is present)
The process can repeat with CH₃Cl:
And further:
And even further:
Step 4: Termination
The chain reaction ends when two free radicals combine to form a stable molecule:
Termination steps remove free radicals from the system, stopping the chain. Since radicals are present in very low concentrations, termination events are rare compared to propagation steps.
Energy Considerations
The overall reaction is exothermic because the bonds formed (C–Cl and H–Cl) release more energy than the bonds broken (C–H and Cl–Cl). However, the initiation step requires energy input (UV light) to break the Cl–Cl bond.
Substitution Reactions
In a substitution reaction, one atom or group in a molecule is replaced by another atom or group. There are two main types:
Nucleophilic Substitution (SN): A nucleophile replaces a leaving group.
Here, OH⁻ (nucleophile) replaces Br⁻ (leaving group).
Electrophilic Substitution (SE): An electrophile replaces a hydrogen atom on an aromatic ring.
Here, Br⁺ (electrophile) replaces a hydrogen on benzene.
Free Radical Substitution: A free radical replaces an atom.
Addition Reactions
In an addition reaction, atoms or groups are added across a double or triple bond, converting it to a single bond. The unsaturation decreases.
Electrophilic Addition: An electrophile adds to an unsaturated bond.
Catalytic Hydrogenation: Hydrogen adds across a double bond using a metal catalyst.
Addition of Halogens:
Elimination Reactions
In an elimination reaction, atoms or groups are removed from adjacent carbons to form a double bond. Unsaturation increases.
Dehydration of Alcohols (E1):
Dehydrohalogenation (E2):
Saytzeff's Rule: In elimination reactions, the more substituted alkene (with more alkyl groups on the double-bonded carbons) is the major product.
Comparison Table
| Feature | Substitution | Addition | Elimination |
|---|---|---|---|
| Saturation | No change | Increases saturation | Increases unsaturation |
| Bonds broken | 1 σ-bond | 1 π-bond | 2 σ-bonds |
| Bonds formed | 1 σ-bond | 2 σ-bonds | 1 π-bond |
| Example | CH₄ + Cl₂ → CH₃Cl + HCl | CH₂=CH₂ + HBr → CH₃CH₂Br | C₂H₅OH → CH₂=CH₂ + H₂O |
Common Mistakes to Avoid
1. Confusing electrophiles and nucleophiles: Electrophiles SEEK electrons (electron-poor); nucleophiles DONATE electrons (electron-rich). Remember: "Electrophile = electron-loving = electron-acceptor."
2. Mixing up homolytic and heterolytic fission: In homolytic fission, each atom gets ONE electron (produces radicals). In heterolytic fission, one atom gets BOTH electrons (produces ions).
3. Confusing +I/–I with +M/–M effects: The inductive effect operates through sigma bonds and is short-range. The resonance effect operates through pi bonds and conjugated systems. Some groups (like –Cl) show –I effect but can show +M effect in certain contexts.
4. Forgetting that the inductive effect diminishes with distance: The inductive effect becomes negligible beyond 3-4 carbon atoms. It is NOT transmitted through the entire molecule like the resonance effect.
5. Mixing up the order of carbocation stability: The correct order is 3° > 2° > 1° > CH₃⁺. More alkyl groups = more +I effect = more stable carbocation. Do NOT reverse this.
6. Omitting the initiation step in free radical mechanisms: Every free radical mechanism must start with an initiation step (bond homolysis). Without it, there are no radicals to begin the chain.
7. Confusing substitution with addition reactions: Substitution REPLACES one group with another (saturation unchanged). Addition ADDS groups across a double bond (saturation increases). The number of products differs.
8. Ignoring Saytzeff's rule in elimination reactions: The more substituted alkene is the major product in elimination. Always check which alkene has more alkyl groups on the double-bonded carbons.
Exam Checklist
- [ ] I can define and identify electrophiles and nucleophiles with examples
- [ ] I can differentiate between homolytic and heterolytic fission
- [ ] I can explain the inductive effect (+I and –I) with examples
- [ ] I can explain the resonance/mesomeric effect (+M and –M) with examples
- [ ] I can compare inductive and resonance effects
- [ ] I can describe the free radical mechanism (initiation, propagation, termination)
- [ ] I can explain hyperconjugation and its role in stability
- [ ] I can write the stability order of carbocations, carbanions, and free radicals
- [ ] I can distinguish substitution, addition, and elimination reactions with examples
- [ ] I can explain the electromeric effect and its temporary nature